This invention relates to sulfur vulcanizable copolymers of ethylene and vinyl acetate. More particularly, it relates to copolymers of ethylene and vinyl acetate which additionally contain activated halogen atoms as sites for sulfur vulcanization, and to the vulcanized elastomers obtained therefrom.
Copolymerization of ethylene and vinyl acetate is well known; see, for example, U.S. Pat. No. 2,703,794 to Roedel. In general, such copolymerization is effected by reaction of ethylene and vinyl acetate in a suitable autoclave under high pressure in the presence of a suitable initiator, such as ferrous sulfate, sodium bisulfite and water. Processes for the continuous copolymerization of ethylene and vinyl acetate are also known; see, for example, U.S. Pat. No. 3,509,115 to French. The copolymers are also readily obtained by copolymerization in the absence of water, e.g. in the presence of benzene, as described by Raetzsch and Musche, Plaste and Kautschuk, 21 (1), 16-18 (1974) and by Roedel, U.S. Pat. No. 2,519,791 and Hanford, U.S. Pat. No. 2,396,785 (Example 16). Furthermore, a comprehensive survey of ethylene-vinyl acetate copolymers is presented in "Ethylene and Its Industrial Derivatives", S.A. Miller, Edit., published by Ernest Benn, London, 1969, particularly pages 437-475, and in "Encyclopedia of Polymer Technology", vol. 6, pages 387-420, published by John Wiley, New York, 1967.
Copolymers of ethylene and vinyl acetate are known to be amorphous, flexible, relatively transparent products, particularly when the proportion of vinyl acetate varies between 20 and 60 weight percent. These copolymers have found substantial commercial use, especially in the manufacture of water- and grease-resistant coatings for paper, modifiers for waxes, and in hot melt adhesives.
Attempts have been made to obtain cured elastomers from ethylene-vinyl acetate copolymers and, while it is possible to cure them, i.e. to provide covalent cross-links between adjacent substantially linear or branched chains, heretofore such cures have required the use of peroxides. One such example is shown by Madge, Chem. & Ind., 1811 (1962).
Peroxide curing is regarded as an undesirable technique by the rubber industry, as opposed to vulcanization, generally regarded as curing in the presence of sulfur or a sulfur donor. Operating hazards are often associated with the use of peroxides; they frequently have an objectionable odor, and more seriously, they are not readily adaptable to the broad formulation techniques developed for typical sulfur-based vulcanization systems. For these and other reasons, ethylene-vinyl acetate copolymers have found only limited use as elastomers despite certain potentially attractive properties, such as good oil resistanc,e low temperature flexibility, and oxidation resistance. resistance,
Accordingly, it is an object of the present invention to provide sulfur vulcanizable copolymers consisting predominantly of ethylene and vinyl acetate repeating units. It is a further object to provide ethylene-vinyl acetate copolymers containing minor amounts of a comonomer having activated halogen atoms as vulcanization sites or ethylene-vinyl acetate copolymers chemically modified to provide such sites. It is yet another object to provide vulcanized elastomers derived from these copolymers by vulcanization with sulfur or sulfur donors.